Ethylene is one of five well-established plant hormones. It mediates a diverse array of plant responses including fruit ripening, leaf abscission and flower senescence.
The pathway for ethylene biosynthesis has been established (FIG. 1). Methionine is converted to ethylene with S-adenylmethionine (SAM) and 1-aminocyclopropane-1-carboxylic acid (ACC) as intermediates. The production of ACC from SAM is catalyzed by the enzyme ACC synthase. Physiological analysis has suggested that this is the key regulatory step in the pathway, (Kende, Plant Physiol., 91:1-4 (1989). This enzyme has been cloned from several sources (Sato et al., PNAS, (USA) 86:6621 (1989); Van Der Straeten et al., PNAS, (USA) 87:4859-4863 (1990); Nakajima et al., Plant Cell Physiol. 29:989 (1990). The conversion of ACC to ethylene is catalyzed by ethylene forming enzyme (EFE), which has been recently cloned (Spanu et al., EMBO J 10:2007 (1991). Aminoethoxy-vinylglycine (AVG) and .alpha.-aminoisobutyric acid (AIB) have been shown to inhibit ACC synthase and EFE respectively. Ethylene binding is inhibited non-competitively by silver, and competitively by several compounds, the most effective of which is trans-cyclooctane. ACC synthase is encoded by a highly divergent gene family in tomato and Arabidopsis (Theologis, A., Cell 70:181 (1992)). ACC oxidase, which converts ACC to ethylene, is expressed constitutively in most tissues (Yang et al., Ann. Rev. Plant Physiol. 35:155 (1984)), but is induced during fruit ripening (Gray, 1992). It has been shown to be a dioxygenase belonging to the Fe2+/ascorbate oxidase superfamily (McGarvey et al., Plant Physiol. 98:554 (1992)).
Etiolated dicotyledonous seedlings are normally highly elongated and display an apical arch-shaped structure at the terminal part of the shoot axis; the apical hook. The effect of ethylene on dark grown seedlings, the triple response, was first described in peas by Neljubow in 1901, Neljubow, D., Pflanzen Beih. Bot. Zentralb. 10: 128 (1901). In Arabidopsis, a typical triple response consists of a shortening and radial swelling of the hypocotyl, an inhibition of root elongation and an exaggeration of the curvature of the apical hook (FIG. 2A). Etiolated morphology is dramatically altered by stress conditions which induce ethylene production the ethylene-induced "triple response" may provide the seedling with additional strength required for penetration of compact soils, see Harpham et al., Annals of Bot. 68:55 (1991). Ethylene may also be important for other stress responses. ACC synthase gene expression and ethylene production is induced by many types of biological and physical stress, such as wounding and pathogen infection, see Boller, T., in The Plant Hormone Ethylene, A. K. Mattoo and J. C. Suttle eds., 293-314 (1991), CRC Press, Inc. Boca Raton and Yu, Y. et al., Plant Phys. 63:589 (1979).
A collection of mutants affected in this response has been isolated. One class, the ein mutants (ethylene insensitive), are completely insensitive to ethylene. A second class of mutants are affected in only subset of the seeding responses. The hlsl mutant (hookless) completely lacks an apical hook either in the presence or absence of ethylene.
Constitutive hormone response mutants have been useful in elucidating mechanisms that underlie other hormone-mediated responses (e.g. yeast mating factor, Blinder et al., Cell 56:479 (1989)). Despite the information known about ethylene biosynthesis, how plants perceive and transduce hormone signals is almost completely unknown. While many of the components found in animal signal transduction chains have been found in plants, including kinases, and G proteins, no definitive correlation of these signal transducers with any hormone signal has been established. Elucidation of the complex role of these signal molecules would be greatly aided by the isolation of gene mutations which are affected in different steps in the signal transduction pathway.
The present invention addresses these important needs. A novel Arabidopsis thaliana mutant has been identified that constitutively exhibits seedling and adult ethylene responses in the absence of the hormone. The constitutive triple response (ctr) mutants display the "ethylene" phenotypes even in the presence of inhibitors of ethylene biosynthesis or receptor binding. ctr1 has a dramatically altered adult morphology that can be phenocopied in wild-type plants by growth in 1 ppm ethylene. Seedling and adult ctr1 plants show high-level constitutive expression of mRNAs for several ethylene inducible genes. Genetic, molecular and biochemical characterization of the CTR1 gene and protein product is set forth in the present invention. Genetic characterization of the interactions among modulatory components of the ethylene action pathway will provide insight into how plant hormones function. Thus, the quality, quantity and longevity of food, such as fruits and vegetables, and other plant products such as flowers, will be improved for market in both developed and undeveloped countries.